Conventionally, one of the said parts comprises two guide rings which are arranged on either side of a damper plate which is part of the other one of the two said parts. The damper plate or the guide rings are secured to a hub for rotation therewith, with, optionally, a facility for an initial clearance to be taken up before rotation witch the hub takes place.
As is well known, in engines of the reciprocating type, in which the power output is delivered in the form of periodic impulses, due to the expansion of a gas against pistons which actuate a crankshaft by means of connecting rods, the variations in engine torque and inertia torque arising from the various components of the engine which are set in reciprocating motion usually make it necessary to provide a flywheel in order to smooth out the speed.
In order to reduce the vibrations which occur, in particular in the region of the pressure plate of the clutch when the latter is being disengaged, the published patent application No. DE 3 609 048A of the Federal Republic of Germany proposes to equip the flywheel with a dynamic vibration damper. This damper is effective in the frequency range which includes the resonant frequency of the pressure plate when the latter is not in gripping engagement with the friction disc of the clutch.
Such an arrangement is well adapted for damping out engine vibrations, and in particular the axial vibrations of the crankshaft. However, it is not entirely satisfactory: it does not allow the natural frequencies in torsion of the whole of the kinematic transmission train which couples the engine to the road wheels, and which includes the gearbox, to be suppressed. Accordingly, it does not allow the critical vibrations which occur in the region of the resonant frequency of the transmission train to be eliminated in certain vehicles at given engine speeds when the clutch is engaged. In addition, such a damper is arranged to extend over a circumference of large diameter, so that it is necessary to provide some suitable means preventing its mass from flying off under the effect of centrifugal force. Besides which, this arrangement requires modification of the reaction plate or flywheel of the clutch.
It has also been proposed, in United Kingdom published patent application GB 2 157 398A, to suppress the vibrations when the engine is in an idling mode, that is to say when the transmission is not under load, this being achieved by means of a mass which is welded on to a support mounted on the hub by means of a friction coupling. This particular arrangement is not adapted to remove the critical vibrations which occur under load when the clutch is engaged, in certain vehicles in the vicinity of a resonant frequency, for example at engine speeds of 1500 to 2800 rev/min which are lower than the maximum engine speed.
In order to overcome this problem, it might be thought possible to increase the mass of the reaction plate. This is however not satisfactory, since it leads to an increase in fuel consumption of the vehicle and to a reduction in performance, in terms of its ability to accelerate.
It could also be supposed that the engine torque might be reduced. This is no more satisfactory.